Method for providing back-pressure for a fuel cell stack

ABSTRACT

A method for controlling the pressure within a fuel cell stack to control the stack relative humidity. In one embodiment, a two-position valve receiving the cathode exhaust is switchable between a fully open and a fully closed position, where the valve is opened when the fuel cell system is operating at a low operation temperature and the valve is closed when the fuel cell system is operating at a high operation temperature. A fixed restriction valve is provided in parallel with the two-position valve so that when the two-position valve is fully closed, the proper amount of pressure is provided at the cathode output. In another embodiment, the two-position valve employs leak paths so that when the two-position valve is in the closed position, the cathode exhaust gas can still flow through.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. Patent applicationSer. No. 10/785,654, filed Feb. 24, 2004 now U.S. Pat. No. 7,235,318 andtitled “Fuel Cell System Back-Pressure Control with a Discrete Valve.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method for controlling therelative humidity in a fuel cell stack and, more particularly, to amethod for controlling the relative humidity in a fuel cell stack byselectively opening and closing a discrete two-position valve at thecathode exhaust of the stack to control stack pressure.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. A hydrogen fuel cellis an electrochemical device that includes an anode and a cathode withan electrolyte therebetween. The anode receives hydrogen gas and thecathode receives oxygen or air. The hydrogen gas is dissociated in theanode to generate free protons and electrons. The protons pass throughthe electrolyte to the cathode. The protons react with the oxygen andthe electrons in the cathode to generate water. The electrons from theanode cannot pass through the electrolyte, and thus are directed througha load to perform work before being sent to the cathode.

Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell forvehicles. The PEMFC generally includes a solid polymer electrolyteproton conducting membrane, such as a perfluorosulfonic acid membrane.The anode and cathode typically include finely divided catalyticparticles, usually platinum (Pt), supported on carbon particles andmixed with an ionomer. The catalytic mixture is deposited on opposingsides of the membrane. The combination of the anode catalytic mixture,the cathode catalytic mixture and the membrane define a membraneelectrode assembly (MEA).

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. The fuel cell stack receives a cathodereactant gas, typically a flow of air forced through the stack by acompressor. Not all of the oxygen is consumed by the stack and some ofthe air is output as a cathode exhaust gas that may include water as astack by-product. The fuel cell stack also receives an anode hydrogenreactant gas that flows into the anode side of the stack. The stack alsoincludes flow channels through which a cooling fluid flows.

The fuel cell stack includes a series of bipolar plates positionedbetween the several MEAs in the stack, where the bipolar plates and theMEAs are positioned between two end plates. The bipolar plates includean anode side and a cathode side for adjacent fuel cells in the stack.Anode gas flow channels are provided on the anode side of the bipolarplates that allow the anode reactant gas to flow to the respective MEA.Cathode gas flow channels are provided on the cathode side of thebipolar plates that allow the cathode reactant gas to flow to therespective MEA. One end plate includes anode gas flow channels, and theother end plate includes cathode gas flow channels. The bipolar platesand end plates are made of a conductive material, such as stainlesssteel or a conductive composite. The end plates conduct the electricitygenerated by the fuel cells out of the stack. The bipolar plates alsoinclude flow channels through which a cooling fluid flows.

The humidity or wetness of the membranes in a fuel cell stack is animportant design criteria for effective stack operation. Too much waterwithin the stack acts to prevent the oxygen in the cathode input gasfrom reaching the catalyst on the cathodes. Too little water within thestack causes the stack membranes to dry out and become more susceptibleto cracking and other damage. The more current that the stack generates,the more water is generated as a by-product of the electrochemicalprocess. However, the more air that is forced through the stack by thecompressor to provide more current, the more the stack membranes dryout. Typically, the stack has a 110% relative humidity during its mostefficient operation. For 110% relative humidity, the exhaust gas issaturated 100%, and also includes a little bit of excess water.

Another factor that affects the stack relative humidity is stacktemperature. As the stack temperature increases, the stack's ability tohold water in the vapor state also increases making it more difficult tomaintain a desired stack relative humidity because more water isrequired to do so. Another factor that affects stack relative humidityis the stack pressure. As the pressure in the stack increases, theability of the stack to hold water in the vapor state decreases. Thus,one of the most commonly used techniques to control cathode relativehumidity is to control the fuel cell system pressure and temperature.

Fuel cell systems must reject waste heat. A fuel cell system willinclude a thermal coolant sub-system that removes heat from the stack sothat it operates at its desired operating temperature. The heatedcoolant from the stack is directed to a radiator that reduces thetemperature of the coolant so that it can be returned to the stack toremove the stack waste heat. The size of the radiator limits how muchheat can be removed from the coolant.

The amount of waste heat that the coolant sub-system can reject isdirectly proportional to the operating temperature of the fuel cellsystem. If the system is able to operate at a higher temperature, asmaller radiator can be employed to remove the heat, thus conservingspace. Unfortunately, a higher operating temperature requires a highersystem pressure to keep the stack relative humidity at the desiredlevel. In other words, as the temperature of the stack rises, itsability to hold water increases, thus requiring more water to meet thedesired relative humidity. The higher pressure cancels the effect of thehigher temperature on relative humidity. At higher operatingtemperatures, however, the stack may not produce enough water to meetthe required relative humidity.

Because the size of the radiator is limited in a vehicle, a fuel cellsystem typically must operate at higher temperatures. Therefore, itbecomes necessary to increase the pressure of the fuel cell stack sothat more water is held therein to meet the desired relative humidity.However, high cathode pressures require larger amounts of compressorpower, which results in a reduction in system efficiency.

Two approaches are known in the art to control fuel cell systempressure. One known approach is to employ a fixed orifice at the cathodeexhaust output. Particularly, for high temperature applications thecathode output orifice is sized to provide a sufficient back-pressure tomeet the relative humidity requirements at a maximum system temperature.However, the output orifice also causes high system pressure at lowoutput power, and thus, the fuel cell system efficiency will sufferbecause of the higher compressor parasitic power.

For low temperature applications, where the thermal sub-system size isnot critical, the cathode output orifice is sized to provide a nearlyzero pressure drop. This allows the fuel cell system to run with lowcompressor parasitic losses, and is therefore efficient over the entireoperating range. However, this fuel cell system will be large as aresult of the large radiator required to reject the low-grade heat.

Modeling results have shown that a fixed orifice is capable of reducingthe flow and pressure of the cathode exhaust gas without overloading thethermal sub-system. As a fuel cell power module decreases in flow andpower, the amount of the waste heat the radiator has to dissipate willdecrease. The amount of waste heat a radiator can dissipate isproportional to the temperature difference between the coolant and theambient air. This value is often represented by Q/ITD, where Q is thewaste heat and ITD is the initial temperature difference between thecoolant and the air. Radiators are sized for a maximum Q/ITD at maximumpower. Therefore, as the fuel cell turns down in flow and power, thewaste heat requirement is lowered, which allows the operatingtemperature to be lowered. This allows the operating pressure to belowered.

FIG. 1 is a graph with cathode input air flow on the horizontal axis andthe required compressor pressure on the vertical axis showing thepossible compressor delivery pressure for a fuel cell system employingonly a fixed orifice to control system pressure. Graph line 40 shows theoperation curve of the system for flow versus pressure for a highpressure drop at the fixed orifice, and graph line 42 shows theoperation curve for flow versus pressure for a near zero pressure dropat the fixed output orifice. The fuel cell system will operate on one ofthe graph lines 40 or 42, regardless of the system operatingtemperature. The high pressure graph line 40 wastes energy at part powerconditions, and the low pressure graph line 42 can only meet humidityrequirements at low temperatures.

Another known approach is to use active cathode pressure control. Thishas been done in the past with high-resolution control valves. Thesehigh-resolution control valves offer many discrete valve open positionsor an analog control where fluid flow through the valve can be set atany desirable location. The position of the valve is determined by thecurrent operating temperature of the system and the amount of water thatis being generated by the stack to provide a calculation of whatpressure is necessary to provide a 110% stack relative humidity.Further, it is necessary to provide a safety device that prevents thevalve from failing in the closed position, which could causecatastrophic stack failure from high pressure. While this approachoffers a good solution that attempts to optimize system efficiency overits entire operating range, the control valve and support software arehigh cost components. Further, in the majority of the operatingconditions of a fuel cell system, this level of control is unnecessary.

FIG. 2 is a graph with cathode input air flow on the horizontal axis andthe required compressor pressure on the vertical axis showing thepossible compressor delivery pressures for a fuel cell system employinga high resolution back-pressure valve. Graph line 44 shows how thecathode pressure can be directly controlled independent of flow toprovide the desired relative humidity. At a constant low temperatureoperation, the control valve is wide open so that the pressure drop atthe cathode exhaust is near zero. As the flow increases, the systemtemperature increases, and the valve will be systematically closed toprovide the desired fuel cell back-pressure to control the relativehumidity.

For fuel cell systems having a high resolution back-pressure controlvalve, the valve position is changed as a function of coolanttemperature in order to maintain the desired relative humidity. However,the pressure drop across a fixed orifice is a function of the fluidvelocity, fluid viscosity and orifice shape. Because the orifice shapecannot be changed, the cathode pressure and flow rate cannot beindependently controlled. If the fixed orifice is sized to meet thehumidity requirements at the maximum temperature, maximum flow andmaximum Q/ITD point, it will provide sufficient humidity control in turndown. The minimum pressure required to maintain the desired humidity isgiven by operating at the minimum temperature possible, whilemaintaining a maximum Q/ITD condition in turn down. This requiredpressure is less than the pressure provided by the pressure drop acrossa fixed orifice in turn down when sized at the maximum point.

A system model was constructed where the maximum allowable Q/ITD was setat maximum power and flow. As the module turned down in flow and power,the coolant temperature was lowered only as much as possible withoutexceeding the maximum Q/ITD. The system back-pressure requirement wasthen calculated in order to meet the required humidity level. This givesa flow versus pressure drop curve that is required of the back-pressurevalve in order not to exceed the maximum Q/ITD limit. FIG. 3 is a graphwith cathode input air flow on the horizontal axis and back-pressurevalve pressure drop on the vertical axis, where graph line 46 shows thisrelationship.

The model was re-run using a fixed area orifice for back-pressurecontrol. The orifice was modeled based on a sharp edge orifice that wassized to meet the desired humidity at the maximum flow and temperaturecondition. The pressure drop through a sharp edge orifice isproportional to the square of the fluid velocity. Under this assumption,a flow versus pressure drop curve was generated for the system using thefixed area orifice, as shown by graph line 48 in FIG. 3. This modelingshows that the pressure drop for the fixed area orifice meets or exceedsthe pressure drop requirements to achieve the desired humidity at steadystate turn down points. The fixed area orifice will cause the fuel cellto be slightly over pressurized at part power conditions, which willgive a slightly higher than optimal humidity. This will cause a slightsystem efficiency hit, but is not damaging to stack durability. If thefixed orifice pressure drop was less than the pressure drop required tomeet the Q/ITD limit, then the humidity would be lower than desired andthe stack may be damaged.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a method isdisclosed for controlling the back pressure of a fuel cell stack thatuses a discrete two-position valve at the cathode exhaust gas output. Inone embodiment, the discrete valve is switchable between a fully openand a fully closed position, where the fully open position is used whenthe fuel cell system is operating at a low operation temperature and thefully closed position is used when the fuel cell stack is operating at ahigh operation temperature. A fixed restriction valve is provided inparallel with the discrete valve so that when the discrete valve isclosed, the proper amount of back-pressure is provided at the cathodeoutput to meet the relative humidity requirements.

In another embodiment, the discrete two-position valve employs sizedleak paths so that when the discrete valve is in the closed position,some of the cathode exhaust gas can still flow through under highpressure. In this embodiment, the fixed restriction valve is notnecessary.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph with cathode input air flow on the horizontal axis andcompressor pressure on the vertical axis showing the flow and pressurerelationship for a fuel cell system employing passive back-pressurecontrol using two different fixed orifices;

FIG. 2 is a graph with cathode input air flow on the horizontal axis andcompressor pressure on the vertical axis showing the flow and pressurerelationship for a fuel cell system employing active back-pressurecontrol using a high resolution control valve;

FIG. 3 is a graph with cathode input air flow on the horizontal axis andback-pressure valve pressure drop on the vertical axis for a sharp edgefixed area orifice valve and a constant Q/ITD high resolutionback-pressure valve;

FIG. 4 is a block diagram of a fuel cell system employing a discretetwo-position valve in parallel with a fixed orifice valve forcontrolling cathode exhaust gas pressure and stack relative humidity,according to an embodiment of the present invention;

FIG. 5 is a graph with cathode input air flow on the horizontal axis andcompressor pressure on the vertical axis showing the flow and pressurerelationship for the fuel cell system shown in FIG. 4; and

FIG. 6 is a block diagram of a fuel cell system employing a discretetwo-position valve including leak paths for controlling the cathodeexhaust gas pressure and stack relative humidity, according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa method for controlling the relative humidity in a fuel cell stackusing a discrete two-position cathode exhaust gas valve is merelyexemplary in nature, and is in no way intended to limit the invention orits applications or uses.

FIG. 4 is a block diagram of a fuel cell system 10 employing a fuel cellpower module (FCPM) 12 including a fuel cell stack, according to anembodiment of the present invention. The system 10 includes a compressor14 that receives an inlet air flow, and compresses the air to a pressuresuitable to drive the appropriate amount of air through the FCPM 12 tomeet the power output of the system 10. The FCPM 12 can be any suitableFCPM for a fuel cell system as discussed herein, and its description isnot necessary for a proper understanding of the invention.

The FCPM 12 outputs a cathode exhaust gas on an output line 16. Thecathode exhaust gas is applied to a discrete two-position valve 18 and afixed valve 20 that are in parallel. When the discrete valve 18 is open,the pressure drop at the cathode exhaust gas output will be as low aspossible for the system 10, where the exhaust gas can flow through boththe valves 18 and 20. When the valve 18 is closed, a greater pressuredrop will occur across the orifice of the fixed valve 20. The orifice inthe fixed valve 20 is sized for the maximum required pressure drop atmaximum flow and operating temperature so that the system pressure willallow the system 10 to meet the necessary relative humidity requirementsat high temperature operation. A controller 22 provides a control signalto open or close the valve 18 at the appropriate time depending onsystem parameters, including system operating temperature.

As discussed herein, the discrete valve 18 is a two-position valve. Inone embodiment, one position is a fully open position and the otherposition is a fully closed position. In another embodiment, one positionis a fully open position and the other position is a mostly closedposition providing a minimal flow therethrough. However, the discretevalve 18 is not a sophisticated, multi-positional control valve as wasused in the prior art that has many valve positions between the fullyopen position and the fully closed position.

The majority of time, the fuel cell system 10 will operate at lowtemperature, and therefore will require low back-pressure. In this mode,the discrete valve 18 will be in the open position. As the system loadincreases, thermal management sub-systems (system coolant) will becomesaturated with low-grade waste heat. The fuel cell system 10 will beable to reject the required amount of waste heat through the thermalsub-system at the low operating temperature. Once the fuel cell stacksaturates the thermal sub-system, the temperature of the fuel cell stackwill rise. When the system temperature rises above the point where thedesired stack relative humidity can no longer be achieved without usingsystem back-pressure, the controller 22 closes the discrete valve 18.This will force the cathode exhaust gas through the fixed valve 20,causing the fuel cell pressure to rise to a level where it will meet therelative humidity requirements.

FIG. 5 is a graph with cathode input air flow on the horizontal axis andrequired compressor pressure on the vertical axis showing control curvesfor both the discrete valve 18 and the fixed valve 20. Particularly,when the system 10 is operating at low temperature and the valve 18 isopen, the compressor pressure will follow graph line 26 as the currentoutput increases. In this low-temperature mode, the operatingtemperature of the system 10 will eventually reach a temperature wherethe radiator can no longer remove the waste heat so that the systemoperates at the desired temperature. At that time, the controller 22will close the discrete valve 18, and the system 10 will switch to thehigh-temperature mode where the pressure is increased to provide thedesired relative humidity at the higher operating temperature, asrepresented by graph line 28.

The controller 22 includes suitable software that prevents the discretevalve 18 from cycling in an unstable manner when the system 10 reachesthe temperature that will cause the valve 18 to close. Particularly,when the controller 22 provides a valve control signal to switch thevalve 18, the controller 22 will wait at least some predetermined periodof time to allow the system to stabilize before switching the valve 18to the previous valve position. The switch point for the valve 18 shouldhave this small hysteresis built into the control. For example, if thethermal sub-system limit is determined by the speed of the radiator fan,then the valve 18 can close at 98% of full rated fan speed. The valve 18could open based on 95% of the full rated fan speed. This prevents thevalve 18 from opening and shutting rapidly while the system 10 sits atthe transition point.

FIG. 6 is a block diagram of a fuel cell system 32 similar to the fuelcell system 10 discussed above, where like elements are identified bythe same reference number, according to another embodiment of thepresent invention. In this embodiment, the valves 18 and 20 have beenreplaced by a single valve 34 that also is a discrete two-positionvalve, but provides flow in the closed or high-pressure position. Whenthe controller 22 senses that the operating temperature of the FCPM 12is at a predetermined temperature, and the pressure of the FCPM 12 mustbe raised to satisfy the desired relative humidity, the controller 22provides a signal to close the valve 34. However, the valve 34 stillincludes specifically sized flow-through orifices or leak paths thatallows the cathode exhaust gas to flow therethrough under high-pressurein the same manner as the fixed valve 20.

Because the electrodes in the fuel cell stack of the FCPM 12 do not likeabrupt pressure changes, the valves 18 and 34 are opened and closedslowly by the controller 22 to prevent quick pressure changes within theFCPM 12. If the valves 18 and 34 move too rapidly, the disturbance tothe system 10 may cause air control errors or high delta pressuresbetween the cathode and anode sides of the stack membrane. A valve witha 500 ms transition time makes the system control more accurate. Thevalves 18 and 34 could include a dash-pot that is mechanically builtinto the valve 18 or 34, or the transition could be performedelectrically. If electrical, the solenoid of the valves 18 or 34 couldbe made with a high inductance, slowing the valve down when thecontroller's discrete output driver switches. This also helps reduce theelectromagnetic compatibility of the systems 10 and 32 by loweringdi/dt.

The use of the two-position valves 18 and 34 has significant advantagesover the high-resolution valves employed in the prior art. Particularly,a discrete solenoid valve costs considerably less than an analog motor;a discrete output driver costs considerable less than an analog driver;a discrete valve eliminates the need for a positioned feedback on thevalve; a discrete valve requires lower machine tolerances, making itcheaper to manufacture; and a discrete valve eliminates the need for astack output cathode pressure sensor.

The use of the discrete valves 18 and 34 also provides improvedperformance over the active back-pressure systems. Particularly, thediscrete system has reduced actuator power requirements because thevalve no longer needs to fail in the open position. Because fuel cellsmust vent to atmosphere at shut down, the valves 18 and 34 can fail inthe closed position and still vent the system. Further, the discretesystem has more designed flexibility because it doesn't necessarilyrequire a fail open spring. Also, the discrete system may have moretolerance to freezing because the valve doesn't need a completely closedseal that can get stuck shut from icing.

The present invention also has a lower system cost relative to a fuelcell system having a back-pressure orifice. Particularly, the presentinvention provides reduced compressor motor power, therefore reducingthe motor and controller size and cost.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. A method for controlling the relative humidity in a fuel cell stack, said method comprising: applying a compressed air flow to a cathode input of the fuel cell stack; and controlling the pressure within the fuel cell stack by controlling the back-pressure of a cathode exhaust gas flow from the fuel cell stack, wherein controlling the pressure within the fuel cell stack includes directing the exhaust gas flow through a two-position valve having a first position for providing a low back-pressure if the operating temperature of the stack is below a predetermined temperature and a second position providing a high back-pressure if the operating temperature of the stack rises above the predetermined temperature.
 2. The method according to claim 1 wherein controlling the pressure within the fuel cell stack further includes providing a fixed restriction valve in parallel with the two-position valve so that the fixed restriction valve provides the high back-pressure when the two-position valve is in the second position.
 3. The method according to claim 1 wherein controlling the pressure within the fuel cell stack includes providing leak paths in the two-position valve to provide the high back-pressure when the two-position valve is in the second position.
 4. The method according to claim 1 wherein controlling the pressure within the fuel cell stack includes preventing rapid switching of the two-position valve between the first position and the second position.
 5. The method according to claim 1 wherein the two-position valve has a relatively slow transition time so as to prevent rapid changes between the open and closed position.
 6. The method according to claim 5 wherein the two-position valve has about a 500 ms transition time.
 7. The method according to claim 5 wherein the slow transition time is provided by one of a mechanical dash-pot or electrical control.
 8. The method according to claim 1 wherein the fuel cell stack is on a vehicle.
 9. A method for controlling the relative humidity in a fuel cell stack, said method comprising: applying a compressed air flow to a cathode input of the fuel cell stack; and controlling the pressure within the fuel cell stack by controlling the back-pressure of a cathode exhaust gas flow from the fuel cell stack, wherein controlling the pressure within the fuel cell stack includes directing the exhaust gas flow through a two-position valve having a first position for providing a low back-pressure if the operating temperature of the system is below a predetermined temperature and a second position providing a high back-pressure if the operating temperature of the stack rises above the predetermined temperature, and wherein a fixed restriction valve in parallel with the two-position valve provides the high back-pressure when the two-position valve is in the second position.
 10. The method according to claim 9 wherein controlling the pressure within the fuel cell stack includes preventing rapid switching of the two-position valve between the first position and the second position.
 11. The method according to claim 9 wherein the two-position valve has a relatively slow transition time so as to prevent rapid changes between the open and closed position.
 12. The method according to claim 11 wherein the two-position valve has about a 500 ms transition time.
 13. The method according to claim 11 wherein the slow transition time is provided by one of a mechanical dash-pot or electrical control.
 14. A method for controlling the relative humidity in a fuel cell stack, said method comprising: applying a compressed air flow to a cathode input of a fuel cell stack; and controlling the pressure within the fuel cell stack by controlling the back-pressure of a cathode exhaust gas flow from the fuel cell stack, wherein controlling the pressure within the fuel cell stack includes directing the exhaust gas flow through a two-position valve having a first position for providing a low back-pressure if the operating temperature of the stack is below a predetermined temperature and a second position providing a high back-pressure if the operating temperature of the stack rises above the predetermined temperature, and wherein the two-position valve includes leak paths that provide the high back-pressure when the two-position valve is in the second position.
 15. The method according to claim 14 wherein controlling the pressure within the fuel cell stack includes preventing rapid switching of the two-position valve between the first position and the second position.
 16. The method according to claim 14 wherein the two-position valve has a relatively slow transition time so as to prevent rapid changes between the open and closed position.
 17. The method according to claim 16 wherein the two-position valve has about a 500 ms transition time.
 18. The method according to claim 16 wherein the slow transition time is provided by one of a mechanical dash-pot or electrical control. 